Sealing of anodized coatings



United States Patent Ofiice 3,073,760 Patented Jan. 15, 1963 3,073,760 SEALIING F ANQDHZED COATINGS Henry J Wittrock, @pportunity, Wash, assigner to Kaiser Aluminum 6; Chemical Corporation, Oakland, Calif., a corporation of Delaware No Drawing. Filed May 5, I959, Ser. No. 811,014 14- Qlaims. ((Ii. 2 04- -35) This invention relates to protective oxide coatings on aluminum and aluminum base alloys. More particularly, this invention relates to the elimination or substantial reduction of the sensitivity to fingerprinting of oxide coatmgs on aluminum and aluminum base alloys.

For many purposes aluminum surfaces are protected by an oxide coating produced on the surface by exposure as an anode in an acid electrolyte capable of yielding oxygen on electrolysis, such as an aqueous solution of sulfuric acid. The operation of forming such coating is commonly termed anodizing and the aluminum surface thus protected commonly term anodized. To improve the corrosion resistance of such coating, it is common practice to immerse the anodic coating in hot water. Such process possesses certain inherent disadvantages particularly for outdoor conditions such as encountered in architectural applications. In such applications, anodized aluminum metal treated in a conventional hot water bath may show substantial sensitivity to fingerprinting. Anodized aluminum metal articles prepared in a relatively dry climate in accordance with prior art practice may show little evidence of this sensitivity. However, after shipping the material to a relatively humid climate, fingerprints will become evident.

This invention comprises immersing aluminum base metal, which had been previously anodized by conventiona1 methods, in a warm aqueous solution of a salt of carbonic acid and a metal selected from the group consisting of alkali and alkaline earth metals. In a preferred embodiment of this invention, the metal is selected from the group consisting of sodium and calcium. The pH and temperature are important factors and whether or not the solution is a carbonate or a bicarbonate, or a mixture of both, is a function of the pH of the solution.

Depending on the lustre or brightness of the anodized aluminum metal desired, that is, the degree of specular reflectance, the aluminum metal may be subjected to various pretreatments prior to anodizing. For example, where high lustre or brightness is desired, the base metal may be subjected to conventional polishing or brightening treatments, e.g. mechanical, chemical or electrochemical. Where it is desired that the ultimate article have a matte or satin appearance, the base metal may be subjected to a suitable etching treatment. If desired, the anodizing coating of the aluminum base article may be colored by conventional practices prior to treatment according to this invention.

A detailed description of a satisfactory procedure embodying the principles of this invention is set forth below, it being understood that the conventional water rinsing operations after the various steps are not recited.

The metal is first cleaned in an inhibited alkaline cleaner. Where a high lustre or brightness of the ultimate colored composite is desired, the metal should be treated according to a suitable bright dip process such as that described in US. Patent 2,719,781. On the other hand, where a matte appearance is desired in the ultimate composite, the material may be subjected to a caustic etch treatment in a solution of 5% sodium hydroxide plus 2% sodium fluoride maintained at a temperature of 160 F. for a 5 minute immersion period. Next, the so-treated metal may be rinsed in a 50% by volume nitric acid solu tion. After such a pretreatment, the metal is anodized in a conventional manner.

Thereafter, the anodic oxide coating on the metal is immersed for a period of time ranging from 5 to 45 minutes in an aqueous solution consisting essentially of a salt having the general formula MX, where M is a metal selected from the group consisting of sodium and calcium, and X is an anion selected from the group consisting of carbonates and bicarbonates, in an amount ranging from .060 gram/liter to 5.0 grams/liter of solution, balance water. This solution is maintained at a pH of from 5.0 to 6.0 and at a temperature from 180 F. to 212 F. Where the metal M is sodium, the preferred range of concentration in grams of salt per liter solution is from 0.126 to 5.0. Where the metal M is calcium, the preferred range of concentration in grams of salt per liter of solution is from 0.076 to 5.0.

In order to establish the superior resistance to fingerprinting of the anodic oxide coatings produced in accordance with this invention, and the composite article coated therewith over those produced by conventional prior art methods, tests described hereinbelow have been conducted.

In these tests samples were fabricated from sheets of commercially pure aluminum (i.e., minimum purity 99.00%) cut into 1 /2 inch by 2 inch rectangles providing a total surface area of 6 sq. ins. per sample.

In these tests all the samples were given the following treatment, it being understood that the conventional cold water rinsing operations after the various steps are not recited:

(1) Cleaned for 5 minutes in a mild inhibited alkaline cleaner of the carbonate, phosphate silicate type at a temperature of 170 F. to a water break-free surface.

(2) Bright dipped according to the teaching of US. Patent 2,719,781.

(3) Rinsed for 15 sees. in 50% by volume nitric acid at room temperature.

(4) Anodized in a 50 gallon rectangular tank equipped with stirring devices and lead cathodes for 60 minutes with direct current in an aqueous sulfuric acid electrolyte containing 15% of sulfuric acid maintained at a temperature of 70 F. The current densities were maintained at 12 est. As used herein the term a.s.f. is an abbreviation of the expression amperes per square foot. In general, the voltages necessary to maintain a given current density vary with the cell and the anodizing conditions, and the required voltage varies as the anodizing progresses. In anodizing the samples employed in these tests, the voltages required to maintain a constant current density of 12 a.s.f. ranged from about 2 to 17 volts.

(5) The aluminum alloy samples treated as above were subsequently immersed for a period of 30 minutes in the solutions indicated in the table below. The solutions were contained in a 4 liter beaker, the pH of all solutions was 5.5 and the temperatures were maintained at 200 F. As employed in the table DW indicates deionized water while TW indicates tap water containing in parts per million 9 sodium, 32 calcium, 14 magnesium, 49 S05, 2 Cl-, 14 co,, 113 HCO;,, 0 hydroxide, balance water having a pH of 8.1. TW Conc. 50% means tap water boiled down to one-half its original volume, while TW Cone. means tap water boiled down to one-tenth its original volume.

(6) All samples were rinsed briefly in distilled water and dried.

All the samples treated as above described were subjected to the following tests:

(1) One set of samples treated as outlined above and in the table were tested for completeness of sealing by the method described in Standard Method of Test For Sealing Anodically Coated Aluminum, A.S.T.M.v Designation B13645, 1955 Standards, Part 2, page 971. This test consists of applying a spot of Antraquinone Violet R (1 gram in 50 ml. of water) to the anodized surface allowing it to stand for minutes, washing with running water and rubbing with soap and water. If the coating is sealed, no color remains. All samples were sealed.

(2) A second and third set of samples were tested for sensitivity to fingerprinting by impressing a fingerprint on each sample, storing one set of samples for twenty-four hours in a container maintained at a relative humidity of 30%, and storing the other set of samples in a container maintained at a relative humidity of 80%. The samples were then observed without opening the containers. Fingerprinting on the sample is indicated by a in the table, While the space is left blank for those samples which did not show fingerprinting.

Table Results, Humidity Sealing Solution TW Cone: 9oa+roo g./l. NaHCO; TW Cone. 90%+2.00 g./l. NaHCO It is readily seen from the table that coated composite articles produced by the method of this invention as illustrated by samples 25, 7-10, 12, 14 and 15 are superior to the oxide coated composite articles produced by sealing the oxide coatings in water as illustrated by samples 1, 6, 11 and 13 with regard to freedom from sensitivity to fingerprinting.

As used herein the term aluminum is meant to cover high purity aluminum, commercial purity aluminum and aluminum alloys.

It will be understood that various changes, omissions and additions may be made to this invention without departing from the spirit and scope thereof as set forth in the appended claims.

What is claimed is:

1. In the art of treating protective oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the oxide coated aluminum metal in a warm aqueous solution consisting essentially of a salt of carbonic acid and a metal selected from the group consisting of alkali and alkaline earth metals, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

2. In the art of treating protective oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the oxide coated aluminum metal in a warm aqueous solution consisting essentially of a salt having the general formula MX wherein M is a metal selected from the group consisting of sodium and calcium and X is an anion selected from the group consisting of carbonates and bicarbonates, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

3. A method according to claim 2 wherein the salt is sodium bicarbonate.

4. A method according to claim 2 wherein the salt is sodium carbonate.

5. A method according to claim 2 wherein the salt is calcium bicarbonate.

6. A method according to claim 2 wherein the salt is calcium carbonate.

7. In the art of treating protective anodic oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the anodized aluminum metal for a period of time ranging from 5 to 45 minutes in an aqueous solution consisting essentially of a salt having the formula MX wherein M is a metal selected from the group consisting of sodium and calcium and X is an anion selected from the group consisting of carbonates and bicarbonates, said salt being present in an amount ranging from 0.06 to 5.0 g./l. of solution, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from F. to 212 F.

8. In the art of treating protective anodic oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the anodized aluminum metal for 5 to 45 minutes in an aqueous solution consisting essentially of sodium bicarbonate in an amount ranging from 0.126 to 5.0 g./l. of solution, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

9. In the art of treating protective anodic oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the anodized aluminum metal for a period of time ranging from 5 to 45 minutes in an aqueous solution consisting essentially of sodium carbonate in an amount ranging from 0.126 to 5.0 g./l. of solution, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

10. In the art of treating protective anodic oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the anodized aluminum metal for a period of time ranging from 5 to 45 minutes in an aqueous solution consisting essentially of calcium bicarbonate in an amount ranging from 0.076 to 5.0 g./l. of solution, balance water, said solution being maintained at a pH of from 5 .0 to 6.0 and at a temperature of from 180 F. to 212 F.

11. In the art of treating protective anodic oxide coatings on aluminum metal, to substantially reduce their sensitivity to fingerprinting the improvement comprising immersing the anodized aluminum metal for a period of time ranging from 5 to 45 minutes in an aqueous solution consisting essentially of calcium carbonate in an amount ranging from 0.076 to 5.0 g./l. of solution, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

12. A composite article having substantially reduced sensitivity to fingerprinting, said article comprising a base portion of aluminum metal and a protective anodic oxide coating adhering thereto which has been treated by immersing the oxide coated article in an aqueous solution consisting essentially of a salt of carbonic acid and a metal selected from the group consisting of an alkali and alkaline earth metal, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

13. A composite article having substantially reduced sensitivity to fingerprinting, said article comprising a base portion of aluminum metal and a protective anodic oxide coating adhering thereto which has been treated by immersing the oxide coated article in an aqueous solution consisting essentially of a salt having the general formula MX, wherein M is a metal selected from the group consisting of sodium and calcium and X is an anion selected from the group consisting of carbonates and bicarbonates, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

14. A composite article having substantially reduced sensitivity to fingerprinting, said article comprising a base portion of aluminum metal and a protective anodic oxide coating adhering thereto which has been treated by immersing the oxide coated aluminum metal for a period of from 5 to 45 minutes in an aqueous solution consisting essentially of a salt having the general formula MX, wherein M is a metal selected from the group consisting of sodium and calcium and X is an anion selected from the group consisting of carbonates and bicarbonates, in an amount ranging from 0.06 to 5.0 g./1. of solution, balance water, said solution being maintained at a pH of from 5.0 to 6.0 and at a temperature of from 180 F. to 212 F.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Metal Finishing, August 1956, pages 53-58.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No $073,760 January 15, 1963 Henry J, Wittrock' rs in the above numbered pat- It is hereby certified that error appea etters Patent should read as ent requiring correction and that the said L corrected below.

" read anodized line 53, for "anodizing d Anthraquinone "0 Column 1,

for "'Antraquinone" rea column 3, line 3 Signed and sealed this 20th day of August 1963.

(SEAL) Attest:

ERNEST w. SWIDER DAVID LADD Attesting Officer Commissioner of Patents 

1. IN THE ART OF TREATING PROTECTIVE OXIDE COATINGS ON ALUMINIUM METAL, TO SUBSTANTIALLY REDUCE THEIR SENSITIVITY TO FINGERPRINTING THE IMPROVEMENT COMPRISING IMMERSING THE OXIDE COATED ALUMINIUM METAL IN A WARM AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF A SALT OF CARBONIC ACID AND A METAL SELECTED FROM THE GROUP CONSISTING OF ALKALI AND ALKALINE EARTH METALS, BALANCE WATER, SAID SOLUTION BEING MAINTAINED AT A PH OF FROM 5.0 TO 6.0 AND AT A TEMPERATURE OF FROM 180*F. TO 212*F. 